An infrared beam transmitting lens is generally produced by cutting, grinding and polishing a block of silicon single crystal.
On the other hand, a visible beam transmitting lens is produced through the steps of heating and pressing a preform of oxide glass by a molding apparatus. An exemplary molding apparatus which is suitable for this application is illustrated in FIG. 1. This apparatus is also described in U.S. Pat. No. 4,854,958 which is incorporated herein by reference. In the exemplary molding apparatus, a preform of oxide glass 1 is placed in a cavity which is surrounded by an upper mold 2, a lower mold 3 and a bushing 4. The optical surfaces 5 are fabricated in the ends of the molds. Heating is provided by means of an induction heating coil 6, the molds and the glass preform are brought to a temperature at which the glass exhibits a viscosity between 10.sup.8 -10.sup.12 poises, a load is applied to the molds by mold carriers 7 and 8, and the motion of the molds is defined by bushing 4 and frame 9. The temperature of the mold is monitored and controlled by thermocouple 10. In this case, the upper and the lower surfaces of the glass preform are heated to the same temperature and pressed.
Prior methods for producing infrared beam transmitting lenses have often required many processes and expensive hand work. Prior methods for producing visible beam transmitting lenses have often resulted in low production rates when chalcogenide glass lenses having precision optical surfaces of different curvature radii on its both sides have been formed. This is because the viscosity of chalcogenide glass falls more rapidly than that of oxide glass as its temperature increases.